A serializer/deserializer (SERDES) is a transceiver which functions as a critical block component of field programmable gate array (FPGA) products. The SERDES has various operating modes including a normal operating mode and a low power mode. The SERDES toggles between these operating modes in response to signals being received at the receiving end of the SERDES. An out-of-band (OOB) detection circuit is typically implemented within the SERDES for analyzing those received signals for determining which mode of operation should be established for the SERDES.
When the OOB detection circuit fails to detect a “present” signal within a threshold period of time, the OOB detection circuit determines that the SERDES should operate in a low power mode. When the OOB detection circuit does detect a “present” signal within a threshold period of time, the OOB detection circuit determines that the SERDES should operating in a normal operating mode. The signals received by the OOB detection circuit are received as input differential pairs (e.g, a positive signal and a complementary negative signal). The OOB detection circuit then compares the difference in magnitude between the input differential pairs against a threshold value to determine whether an “absent” signal or a “present” signal is detected.
Conventional OOB detection circuits suffer from several disadvantages. The main disadvantage of conventional OOB detection circuits is their inability to detect a “present signal” for both positive and negative polarities. When an input differential pair includes a positive input signal (Inp) and a negative input signal (Inn), the conventional OOB detection circuit is only able to detect a “present” signal where the Inp−Inn is greater than the threshold value. Where the absolute value of Inn−Inp is greater than the threshold value, the conventional OOB detection circuit will detect a false “absent” signal, thereby potentially placing the SERDES in a low-power mode where it should be operating in a normal operating mode.
Additionally, conventional OOB detection circuits suffer from significant power consumption and typically occupy large areas, both of which are undesirable characteristics for a SERDES.